Field of the Invention
The invention relates to a method and device for transferring a trailing edge of a sheet in a reversing device of a sheet-fed rotary printing machine.
In order to print both sides of sheet-like material in a sheet-fed rotary printing machine during a recto/verso or first-form and perfecter printing operation, it is necessary for the sheet to be reversed or turned by a reversing or turning device following a printing of the upper side of the sheet, in order to be able then to print on the reverse side thereof.
The German Published Non-prosecuted Patent Application (DE-OS) 24 51 987 discloses a reversing device with which, during a recto/verso printing operation, the trailing edge of a sheet guided on an impression cylinder is gripped by a suction gripper that is swivellable or pivotable out of the periphery of a downline sheet-guiding cylinder, and is guided into the periphery of the downline sheet-guiding cylinder before passing the transfer center between the two cylinders. In the periphery of the downline sheet-guiding cylinder, the trailing edge of the sheet is then transferred to a further gripper arrangement which transfers the trailing edge of the sheet to a third gripper arrangement, which is likewise arranged in a pivotable or swivellable manner within the periphery. In order to counter a relative movement between the trailing edge of the sheet and the sheet parts which continue to adhere to the impression cylinder, the relative movement leading to errors in register, the suction gripper is guided on a trochoidal path as it is retracted into the periphery of the downline sheet-guiding cylinder. Due to the transfer of the trailing edge of the sheet, which takes place in the interior of the periphery, and due to the complexity of the gear mechanisms used, in-register transfer is not possible when the aforedescribed reversing device is used, in particular at high production speeds. Furthermore, in the case of the aforedescribed device, the sheets, in particular, having greater paper weights, have a tendency, due to centrifugal force, to undergo bulge formation in the trailing part thereof which has been lifted from the impression cylinder by the suction grippers, and this bulge formation forms an additional obstacle to an in-register receiving or takeover operation by the downline gripper devices.
The published German Patent Document DE 38 29 626 C2 describes a sheet-fed rotary printing machine with a reversing device with which, during a recto/verso printing operation, the trailing edge of the sheet is lifted from the upline impression cylinder by a suction gripper that is pivotable or swivellable out of the periphery of a downline sheet-guiding cylinder. When the suction gripper is retracted beneath the periphery of the downline sheet-guiding cylinder, the movement of the suction gripper is controlled by a gear mechanism so that the sheets are subjected to tension or tautened counter to the direction of rotation of the downline sheet-guiding cylinder.
The transfer of the trailing edge of the sheet by the suction grippers within the periphery of the downline sheet-guiding cylinder means that, in the case of the devices according to the German Patent Document DE 38 29 626 C2 and the published Non-prosecuted German Patent Application (DE-OS) 24 51 987, comparatively large relative movements take place between the sheets and the interior gripper devices and, if the grippers and suckers which are used do not follow on, the large relative movements may result, for example, in so-called "picking", because the gripper retaining the leading edge of the sheet remains closed, which, in turn, is associated with serious register-related problems. Furthermore, an irregular gripper closure or an irregular gripper opening, as often occur between the drive side and the operator side of a sheet-fed rotary printing machine due to the elasticity of the gripper shafts, may result in "picking" at discrete points in time between the drive side and the operator side. The causes of the different grips or gripper lockings are dynamic in nature and are brought about, for example, by different gripper-shaft torsion, differences in rotational speed, and so forth. Furthermore, in particular in the case of thick printing materials, a twisting of the sheets which causes register errors may occur. This twisting can be attributed to the fact that the joint grip between the opening of the leading-edge gripper of the impression cylinder and the closure of the trailing-edge gripper, which is usually in the region of approximately 1 to 2 degrees of machine angle, is increased further due to the greater thickness of the printing materials.